The White House is planning to host large parties again for the holidays, only this time, officials are apparently not even bothering to test the guests in advance. Presumably the administration has finally, belatedly realized that testing alone won’t protect people from the virus. Unfortunately, officials still haven’t learned what they needed to, which is not “tests don’t work,” but “testing is a powerful tool with some major limitations.”

If they’d absorbed that lesson, President Trump might have led on an issue where there’s still a whole lot of confusion. Instead, his administration apparently abandoned “test everyone” in favor of “nihilistic shrug.” (Guests at the White House parties, we’re informed, will be served on individual plates from behind plexiglass partitions, which I suppose will at least give them something to lean on if they start feeling ill.)

Meanwhile, Americans remain confused. Take my smart and well-informed friend who asked me, during the White House epidemic in October, whether you could infect people while still testing negative. Or consider all the people who lined up to get a covid test in the week before Thanksgiving so they could holiday indoors with a clean conscience.

So let’s clear things up: Yes, absolutely, you can test negative while you are contagious.

For example, according to studies highlighted on the Abbott Labs website, the test that the White House was using could miss almost 10 percent of active infections. Some studies have put the rate of false negatives even higher. All the other available tests also fail some of the time, though the rates vary from test to test.

The high rate of false negatives means that testing provides the most protection when it’s deployed at the population level. At the group level, it’s only a weak, adjunct tactic to other precautions. And at the individual level, it’s borderline useless.

Start with the individuals: If you think you might have been exposed to covid-19 — for example, by flying home for the holiday — a negative test can’t tell you it’s safe to hang out indoors with your elderly parents. There’s a significant risk that you simply aren’t testing positive yet, and with case fatality rates still extremely high among older age groups, that’s a risk you should take very seriously.

For a larger group that has to meet in person, however, such tests are more useful. Even if the tests fail 1 in 10 times, they’re still reducing the group’s exposure by 90 percent. Meanwhile, other strategies can reduce it further: masks, upgrades to the ventilation system, distancing and hand-washing.

At the group level, we should think of testing the way medieval kings thought about fending off barbarians. They didn’t just throw up a wall and call it a day, because what do you do if the enemy breaches the wall? So they built a layered defense — moats, catapults, boiling oil — plus some soldiers inside the walls. That’s also the best way to hold covid-19 at bay: testing and masks and hand-washing and distancing and good ventilation (or best of all, staying outdoors). Because if you instead try to use testing as a substitute for other safety measures, then eventually the virus is likely to slip through your lone line of defense and wreak mayhem within.

But even better than group testing would be mass testing. Because somewhat paradoxically, even this slipshod strategy might work if we could just test enough people. And with the advent of inexpensive — though somewhat less accurate — home tests, we’re now at a point where we might be able to put a serious dent in transmission just by using tests.

To understand how that might work, imagine that we start with 100,000 infected people, and on average, they’ll each infect three more people if nothing is done. But we don’t do nothing; instead, we start testing that misses 10,000 infections and catches 90,000 of them.

If those people who test positive do the patriotic thing and immediately quarantine themselves, then in the second round of transmission, we would end up with 30,000 infections, instead of 300,000. In the third round, having caught 27,000 of the second round of infections, we end up with 9,000 new infections, instead of 900,000. Repeat that a few more times, and we’ve basically wiped out the novel coronavirus without doing much else.

Of course, it’s not practical to test the whole country at once, and even if it were, many people would refuse. But it’s still true that the more tests we do, the safer we will all be, so long as we all understand what we’re doing. Unfortunately, the White House, which ought to have been leading the way, instead chose the road to disaster — and even now, too many Americans appear to be following in its path.

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